This invention relates to photographic exposure control systems and, more particularly, to an automatic exposure control system for controlling aperture and exposure time interval parameters to accommodate very high ambient light levels incurred, for example, when photographing subjects in brightly illuminated snow and beach settings.
Modern cameras commonly incorporate automatic exposure control systems which employ integrated logic circuits and microprocessors to achieve automated control of substantially all camera operating parameters which contribute to proper exposure of film to a subject or scene to be photographed. Generally, these automatic exposure control systems receive and process input data such as film speed, the brightness level of ambient light reflected from the subject, and camera to subject distance to control the sequential operation of camera components such as a lens focussing mechanism, a shutter and/or aperture setting device, and a flash unit during the interval of time required for the depression of a camera actuating button. Also, such exposure control systems are used with a wide variety of exposure control mechanisms by which shutter interval and aperture size parameters are determined.
An automatic exposure control mechanism employing a scanning blade type shutter which, in operation, varies the aperture size simultaneously with the exposure interval is described in commonly assigned U.S. Pat. No. 3,942,183 entitled "CAMERA WITH PIVOTING BLADES", issued to George D, Whiteside on Mar. 2, 1976. Such scanning blade type shutter mechanisms generally include a pair of counter-reciprocating blade elements each having a primary aperture that traverses the optical axis of the camera during the exposure interval. The primary apertures are shaped so that on overlying one another during countermovement of the blades, an effective exposure aperture value is defined and which increases to a maximum value in a determinant period of time.
Generally, exposure control in such a scanning blade system is provided by a pair of secondary photocell apertures, one aperture in each of the blade elements, which admit scene light to a photocell in correspondence with the scene light admitted to the focal plane during shutter blade movement through an exposure cycle. The output from the photocell is directed to an integrator circuit which triggers upon reaching an integration level related to a desired exposure value or more precisely an aperture-interval value to terminate the exposure interval by returning the shutter blade elements back to their initial scene light blocking position.
Shutter mechanisms of the aforementioned type, and particularly the initial portion of the aperture curve are configured to provide an increased aperture-interval value at ambient light levels of about 800 candles per square foot, to provide an artificially high aperture-interval value in an attempt to reduce subject underexposure in such conditions.
Stated otherwise, within certain constraints, the system is generally designed to provide an aperture-interval value for each scene light level in keeping with, or in tracking relation to the exposure curve of the film to produce an average exposure for each level of scene light. At high light levels, however, based upon assumptions as to scenes commonly photographed, it has been found desirable to modify this tracking relation to provide greater aperture-interval values than that defined solely by the film parameters so as to deliberately produce scene overexposure to thereby offset underexposure of the subject.
Consequently, scanning shutter systems of the above-mentioned type, while providing decreasing aperture-interval values (defined as the function of aperture area and exposure interval) with increasing scene light levels, are configured to produce slightly greater than anticipated primary aperture values or smaller than anticipated secondary aperture values, or a combination of both, during the opening portion of the shutter scanning to provide a slightly larger aperture-interval value during the opening portion thereby producing overexposure of the scene when high scene light levels are encountered.
This increased scene exposure or "kick-up" is limited by the requirements of the blade aperture configuration and the ability of the shutter blades to respond to the electronic system, and therefore results in inadequate adjustment of the exposure when the camera is operated in such high ambient light. In any event, the described system cannot achieve the straightforward automatic exposure program of the present invention in which, above a select high light level, the exposure doubles with a doubling of the scene light.
In addition to the above, different types of photographic systems address the problem of subject underexposure encountered by automatic exposure systems when the ambient light level is high. Known systems, for example, incorporate such arrangements as spot metering of the subject illumination, complex sensing systems using multiple light level detectors and circuitry to compare the light level of the subject to the light level of the scene, or a manually operated trim mechanism which can be utilized to decrease the amount of light impinging upon the photocell to thus increase the effective aperture-interval value.
For example, commonly assigned U.S. Pat. No. 4,423,936 entitled "PHOTOGRAPHIC EXPOSURE CONTROL SYSTEM AND METHOD" issued to Bruce K. Johnson on Jan. 3, 1984 discloses a photographic exposure control system which automatically classifies ambient scene lighting conditions as normal, back-lit or fore-lit and then selects an exposure parameter signal program that is appropriate for the indicated condition. The system monitors a plurality of different portions of the scene area with a multi-sensor array that detects both subject range and ambient light intensity. Ambient light intensity measurements of the subject and non-subject areas are compared to classify the lighting conditions and to select the corresponding signal program which controls operation of the system components to vary the ratio of ambient to artificial light contribution to exposure for the indicated lighting conditions.
Commonly assigned U.S. Pat. No. 4,375,322 entitled "FULL RANGE VARIABLE PROPORTIONAL FILL FLASH SYSTEM" issued to Richard Coppa et al on Mar. 1, 1983 and U.S. Pat. No. 4,329,031 entitled "PHOTOGRAPHIC SYSTEMS FOR VARYING FLASH FIRE APERTURE AS A FUNCTION OF AMBIENT SCENE LIGHT" issued to Peter Carcia et al on May 11, 1982, disclose photographic exposure fill flash control systems which are designed to maintain the same proportional contribution to the film exposure from the ambient and artificial light during a film exposure.
Finally, commonly assigned U.S. Pat. No. 4,315,676 entitled "CAMERA WITH AUTO RANGING FOCUSING AND FLASH FIRE CONTROL" issued to Arthur LaRocque et al on Feb. 16, 1982 and U.S. Pat. No. 4,342,506 entitled "EXPOSURE TRIM MECHANISM FOR CAMERAS" issued to Bruce Johnson et al on Aug. 3, 1982 disclose automatic exposure control systems including manually operated exposure trim mechanisms by which the photographer may adjust the proper exposure settings depending on ambient light conditions. The trim mechanism includes a variable density filter which is moved over a photocell towards an extreme position of least transmittance thereby allowing the least amount of light to reach the photocell in order to increase the exposure interval. This could be utilized by a knowledgeable operator to increase the film exposure when the photographer recognizes the indicated special conditions of high ambient scene light intensity. The above trim mechanism not only provides a longer exposure interval by reducing the response of a photoresponsive cell to ambient light but also increases the tension on a spring mechanism which will accelerate the rate at which the shutter blades move to their open position thereby providing a larger effective aperture.
To some degree, the above mentioned exposure control systems address the problem of underexposure of the principal subject in lighting conditions of high ambient scene light but suffer from the drawbacks of requiring either a complex arrangement of a plurality of photocells and discrimination circuitry, the requirement of increasing the output of an artificial light source above a normal level which has practical limitations, or manually adjusted systems which require a photographer to discriminate between differing scene lighting conditions and make proper exposure adjustments.